This invention relates to a device for the automatic control of the weft yarn feed in air looms.
It is known that the proper working of air looms--in which the weft yarns are inserted into the shed and conveyed through the same by means of air blasts blowing from nozzles suitably arranged upstream of the loom, along the shed and downstream of the loom--greatly depends on the constancy of the physical characteristics of the yarn. In fact, any fluctuations in said characteristics tend, above all, to easily produce a variation in the "flight" time of the weft yarn. Assuming that the setting of looms is optimized for a short "flight", any variations in said physical characteristics of the yarn, leading to unexpected longer "flights", lead to a lengthening of the cycle time with the possibility of machine stoppage. Whereas, if the loom is set for a long "flight" (in practice, one should vary the feed pressure of the main nozzle and/or of the auxiliary nozzles), it is possible to avoid stoppage under the previously specified conditions, but the result is a greater, undesired and unnecessary energy consumption, should the yarn conditions subsequently allow shorter flight times. In any case, there is the possibility of defects appearing in the finished fabric.
On the other hand, maintaining the physical characteristics of a yarn constant is a purely theoretical characteristic, as the said fluctuations practically occur in all yarns, even if to an obviously greater or lesser extent according to the type and quality of the yarn. It is therefore extremely important that the working of air looms should not be limited by these fluctuations and this can be achieved by modifying the operation of the loom based on the characteristics of the yarn being woven, so that the drawbacks deriving from the inconsistency of its physical characteristics may be eliminated or at least greatly reduced.
An attempt in this respect has already been made in the past (see for example the W.O. publication No. 84/02360) by determining, through sensors, the time taken for each yarn turn (or turn section) leaving the feeder to unwind, and by setting, accordingly to the determined values, the operating periods--which may continuously vary--of the single nozzles. This process has however the disadvantage of requiring the insertion of the single wefts at times differing from that programmed for the proper working of the loom and also differing one from the other, which may cause serious defects in the fabric.
Furthermore, due to the technical operating times of the solenoid valves and to the high loom speeds, it is not easy to obtain the desired automatic control.